1. Field of the Invention
The present invention relates to a diffractive element which selectively diffract different wavelengths, to a method of manufacturing the diffractive element, and to an optical pickup device and an optical disc apparatus, which are equipped with the diffractive element.
2. Description of the Related Art
Conventionally, in digital versatile discs (DVDs), recording or reproducing of information is performed by a wavelength λ1 (about 650 nm) of laser light. On the other hand, in compact discs (CDs), recording or reproducing of information is performed by a wavelength λ2 (about 780 nm) of laser light. In optical disc apparatuses which perform recording or reproducing on these two types of discs, a dual-wavelength laser light source which emits two wavelengths of laser light has been used. Diffractive elements with wavelength selectivity which transmit the wavelength λ1 of light for DVDs without any change as one beam without responding thereto and which split the wavelength λ2 of light for CDs (while serving as a diffractive element for this wavelength of light) into three beams have been needed for such a dual-wavelength laser light source. The diffractive elements having wavelength selectivity are exemplified in JP-A-2002-318306, JP-A-2002-360625, and US Publication No. 2004-0094699.
FIG. 24 is a plan view showing a diffractive element according to the related art. FIG. 24A shows that a wavelength λ1 of light for DVDs transmits through the diffractive element, and FIG. 24B shows that a wavelength λ2 of light for CDs transmits through the diffractive element.
As shown in FIG. 24, a first transparent 101 is a substrate made of optical glass, etc. A second member 103 includes a red organic pigment 105 and a resin 103a containing the organic pigment 105, and forms irregular shapes on the first transparent substrate 101. A first member 102 includes a resin 102a which is filled in the irregularities of the second member 103. The first member 102 and the second member 103 constitutes a diffraction grating. The second transparent substrate 104 is a substrate made of optical glass, etc., and protects the first member 102 and the second member 103. The organic pigment 105 has optical absorption in a range having a shorter wavelength than the wavelength λ1, but does not have optical absorption in the wavelength λ1 and the wavelength λ2.
It is known that the refractive index of general materials changes according to wavelengths. It is also known that, when a material absorbs light in a certain wavelength range, the refractive index of the material changes rapidly in a wavelength range having the optical absorption and in a wavelength range in the vicinity thereof. This phenomenon is referred to as abnormal dispersion phenomenon. In Patent Document 1 and Patent Document 2, the abnormal dispersion is given to only the second member 103, using the organic pigment 105. And, by greatly changing the refractive index of the second member 103 in the wavelength λ1 and the wavelength λ2, the refractive index n1 (λ1) of the first member 102 in the wavelength λ1 and the refractive index n2 (λ1) of the second member 103 in the wavelength λ1 are made equal to each other, and the refractive index n1 (λ2) of the first member in the wavelength λ2 and the refractive index n2 (λ2) of the second member in the wavelength λ2 are made different equal to each other. Therefore, the wavelength λ1 of light is transmitted without any change as one beam because there is no difference in refractive index between the first member and the second member and the wavelength λ2 of light (here, the diffractive element serves as a diffractive element for this wavelength of light) is split into three beams because there is any difference in the refractive index. In other words, it is possible to obtain a diffractive element having wavelength selectivity.
Meanwhile, since the pigment is dispersed and suspended in particles in the second member, incident light is dispersed. Therefore, it is difficult to increase the transmittance of the second member including an organic pigment, in the wavelength λ1 of light for DVDs and the wavelength λ2 of light for CDs. Therefore, the light for DVDs and CDs which transmits through the diffractive element may be lost. It has been advanced to cope with miniaturizing the optical disc apparatus or the optical pickup device and increasing its processing speed to a high speed Increasing their speed to a high speed requires increasing the output of laser light to be radiated on an optical disc. Therefore, it is necessary to minimize a loss caused by the diffractive element.